Distortion product emissions, sounds emitted by the ear when it is stimulated by pairs of tones, are widely thought to offer a useful objective measurement of high-frequency hearing loss in humans. However, at these same frequencies, the systems commonly used to measure emissions are subject to large errors in calibrating the stimulus sound pressure levels. This results from the presence in the ear canal of reflections of sound from the eardrum that lead to pronounced spatial nonuniformities in the sound levels. These errors appear to introduce substantial variability in emission data sampled in populations of human subjects at frequencies above 4 kHz. Furthermore, these systems do not generate usable stimulus intensities at the highest frequencies of human hearing. Since the high frequency range has the greatest potential diagnostic value for distortion product emissions, these problems should be addressed quickly so that reliable normative standards can be established for emissions at high frequencies. A probe tube microphone will directly measure the stimulus level near the eardrum. Both acoustic and optical methods will provide the feedback necessary to position the probe at a consistent location. This System will be used to measure audiometric thresholds with the same apparatus used to measure distortion product emissions in a large sample of human subjects with normal hearing. The development of a high-frequency sound source will allow for the first time reliable measurements of distortion product emissions over the entire hearing range. The data collected should be of sufficiently reliability to evaluate critically the potential diagnostic utility of the emission measurements at high frequencies as well as models of transmission of the emissions from the cochlea to the external ear canal. These models in turn can be used to predict the degree of - correlation expected between emission levels and audiometric thresholds.